The luminances of a transmissive board such as an instrument board for a vehicle sometimes varies depending on the position of a light source or the distance from the light source. As a method of equalizing the luminance, the following method has been already known (see the patent document 1 and the patent document 2). The method includes the step of photographing the front side of the transmissive board at the moment of transmissive illumination, then the image data of the board is converted to electric signals, the step of implementing the dots disassembly based on the information of the electric signals to make an original plate for printing, and the step of providing dots to a transmissive board based on the original plate for printing.
Patent Document 1: Japanese Patent Laid-Open No. 60122947
Patent Document 2: Japanese Patent Laid-Open No. 6361117